The increasing prevalence of fiber optic communications systems has created an unprecedented demand for devices for processing optical signals. Planar devices such as optical waveguides, couplers, splitters, and amplifiers, fabricated on planar substrates, like those commonly used for integrated circuits, and configured to receive and process signals from optical fibers are highly desirable. Such devices hold promise for integrated optical and electronic signal processing on a single semiconductor-like substrate.
The basic design of planar optical waveguides and amplifiers is well known, as described, for example in U.S. Pat. Nos. 5,119,460 and 5,563,979 to Bruce et al. U.S. Pat. No. 5,613,995 to Bhandarkar et al., U.S. Pat. No. 5,900,057 to Buchal et al., and U.S. Pat. No. 5,107,538 to Benton et al., to cite only a few. The devices consist, very generally, of a core region, typically bar shaped, of a certain refractive index surrounded by a cladding region of a lower refractive index. In the case of an optical amplifier, the core region contains a certain concentration of a dopant, typically a rare earth ion such as an erbium or praseodymium ion which, when pumped by a laser, fluoresces, for example, in the 1550 nm and 1300 nm wavelength range, respectively, used for optical communication, amplifying the optical signal passing through the core.
As described, for example, in the patents to Bruce et al., to Bhandarkar et al., and to Buchal et al., planar optical devices may be fabricated by process sequences including forming a layer of cladding material on a substrate, forming a layer of core material on the layer of cladding material, patterning the core layer using a photolithographic mask and an etching process to form a core ridge, and covering the core ridge with an upper cladding layer.
To be useful in optical communications systems, devices need to have low loss levels; that is, the intensity and optical quality of optical signals should not be inadvertently degraded by the planar optical devices. Typical systems requirements specify devices with passive insertion losses of less than about 0.2 dB/cm. Induced scattering loss due to surface roughness of the sidewalls of the core region has been identified as a significant loss mechanism in planar optical devices. For example, Foresi et al., in U.S. Pat. No. 5,841,931, report that surface roughness is a dominant source of loss in waveguides having polycrystalline silicon cores and silicon dioxide cladding layers. The relationship between scattering loss and waveguide surface roughness is also treated in publications by Lee et al. (Appl. Phys. Lett. 77, 1617 (2000)) and Ladouceur et al. (IEE Proc. Optoelectron. 141, 242 (1994).)
The etching process used to form the core ridge is understood to be a major source of surface roughness of the core sidewalls of planar waveguides and amplifiers. The paper of Ladouceur et al., for example, demonstrates that for a silica based waveguide, the surface roughness of the etched core is intimately linked with the surface roughness of the mask used in the etching process. Rare earth doped materials, used as core materials in amplifiers, are even harder to etch than undoped materials, adding to the surface roughness problem in optical amplifiers. In particular, alumina and other refractory ceramics, which are hosts for rare earth dopants, take on a precrystalline or segregated condition upon heat treatment for dopant activation. Such a precrystalline condition further aggravates sidewall roughness on etching.
Thus it would be desirable to provide waveguide and amplifier designs and fabrication methods that minimize sidewall roughness, enabling the construction of improved devices with reduced scattering losses.